In the field of image processing, there is often a requirement to change the shape of images. In general, images can be represented as sampled values known as pixels. The shape of an image can be changed by interpolating the pixels from which the image is comprised. This could provide for example a linear stretch of the image, or a shape change determined in accordance with a higher order function. The present invention relates to image processors and image processing methods which operate to perform such interpolation.
In order to facilitate explanation and understanding of the advantages provided by the present invention, an example application will be considered of interpolating an image formed using an image lens, which introduces into the image a chromatic error. As such, it will be appreciated that the present invention also relates to video cameras having image processors which operate to improve the quality of an image represented by a video signal generated by the camera, by interpolating at least part of the image.
Optical imaging equipment such as cameras and light projectors are typically provided with an imaging lens, which focuses light to form an image. For cameras, the imaging lens is provided in order to focus an image falling within the field of view of the lens onto a sensor. For colour cameras, the sensor is typically provided with a dichroic element which serves to divide the colour image formed by the lens into red, green and blue components. For colour cameras in which the image is sensed electronically, such as in the case of digital cameras, camcorders, or television cameras, the red, green and blue components are then sampled in order to produce a colour image signal representing in digital form the sampled red, green and blue components of the colour image. In the case of still image digital cameras, the data represented by the colour image signals are stored in order to be reproduced or processed in some way. In the case of television cameras, the colour signals may be recorded, or communicated to a mixing apparatus where for example the camera is used in a television production studio. Similarly however the camera may be a conventional camera in which the sensor is a film which is exposed to a predetermined amount of light produced from the image focused by the lens.
In the above examples a lens is used in order to focus the image falling within a field of view of the lens. However, lenses do not form a perfect representation of the image falling within the field of view of the lens. This is because optical properties of the lens itself cause distortion in the focused image formed by the lens. One example of such distortion is chromatic aberration. Chromatic aberration arises, for example, from dispersion which is a property of the lens resulting from the refractive index of the material forming the lens, such as glass, differing with wavelength. As a result the quality of the colour image formed by the lens is impaired, particularly at the boundaries of the image, where the chromatic aberration causes greatest error.
It is known from co-pending UK patent applications numbers UK 9823400.8 and UK 9823401.6 to improve the quality of an image represented by a video signal by compensating for the effects of the chromatic aberration. This improvement is effected by interpolating between parts of the image represented within a field of the video signal and corresponding parts of the image represented in a different field. As disclosed in these UK patent applications, interpolation can be performed using frame-based interpolation or field-based interpolation. With frame-based interpolation pixels from two or more fields are used to generate an output pixel, whereas with field-based interpolation, pixels from only one field are used. Frame-based interpolation can provide better spatial resolution, because the interpolation is performed on two interlaced fields in which the sampling rate of the video signal is consistent with the bandwidth of the image frequencies made up from the two interlaced fields. As a result an interpolated image produced from frame-based interpolation does not usually suffer from errors introduced by vertical aliasing within the interpolated images. However, if there is motion present in the part of the image being interpolated, the use of two temporally separated fields (in the frame-based interpolation) can produce an inferior image quality because the image has moved between the fields. As explained in UK patent application No. UK 9823400.8, this can cause double imaging. Therefore although framed-based interpolation provides a better vertical resolution, to avoid double images, frame-based interpolation should not be used where the image contains frequencies in the same direction as the motion.
This is because, for example, if the video signal represents the image using vertical frequencies, frame-based interpolation still provides a superior result than field-based interpolation when the motion is horizontal. Field-based interpolation does not generally produce double images, but has lower vertical resolution, and in addition limitations of the interpolation process can include ‘softness’ in the definition of the image and ringing can be produced which is noticeable to the human eye. Therefore in summary, in the event that there is motion present in the image, then field-based interpolation is preferred, although this can result in a reduction of the image quality as a result of artifacts introduced by vertical aliasing.
It will be appreciated from the foregoing discussion that it remains a technical problem to improve the compensating effects of interpolation to images affected inter alia by chromatic error. However this is but one example application wherein interpolation is applied to the pixels of an image in order to change the shape of the image in some way.